Automatic diaphragm control according to the teachings of the present invention uses a DC voltage obtained from rectification of the video output signal as a control signal to form the optimum picture. A rectifier used to detect the light intensity of the object for pickup is the most important element of the control, since the light measuring mode of the diaphragm control is determined by this recifier.
In addition to the absolute brightness of the light intensity of the object for pickup, the ratio between the brightest portion of the object to be monitored by the video signal and the darkest portion of the object, known as the contrast ratio of this object, is also one of the important factors which must be considered in order to obtain the optimum picture. In the natural world, the contrast ratio reaches a level as high as 10 to 1 even under a cloudy sky and often reaches 100 to 1 under a blue sky. In a television-reproduced picture, on the contrary, the contrast ratio is limited to an order of 30 or 40 to 1. Accordingly, it must be decided whether the bright portion or the dark portion should be used as the main object to be monitored for the measurement of light intensity when an object, in general, simultaneously contains both the bright portion and the dark portion, since it is impossible to accurately measure or reproduce both portions at once.
Theoretically, the light measuring mode most suitable for the case where the darkest portion of the object is light-measured may be referred to as the average light measuring mode while the light measuring mode most suitable for the case where the brightest portion of the object to be monitored is light-measured may be referred to as the peak light measuring mode. As both light measuring modes rely upon the video ouput signal from the television camera for diaphragm control, the two types of light measuring modes as mentioned above have already been proposed. However, both of these well known modes still have problems. Specifically, when the same object to be reproduced as a video signal simultaneously contains both the brightest portion and the darkest portion of the picture as previously described, the average light measuring mode will often make the brighest portion reproduce as a white solid portion without a contrast gradient while the peak light measuring mode will often make the darkest portion, which should be normally discriminable by the camera, indiscriminable. As a consequence, the object to be used for metering must be selected after a careful examination of various factors of a particular object, such as absolute brightness and contrast, so far as the conventional video camera metering device of fixed photometric mode type is concerned.